The present invention relates to an image sensor used for facsimile machines, scanners or the like and a method of manufacturing the same, and more particularly to an image sensor having a wiring structure with a less electrical influence among interconnection wires, and a method of manufacturing the same.
In the conventional image sensors, particularly close-contact type image sensors, there is known an image sensor of the type in which image data of documents, for example, is projected onto the sensor in a one-to-one correspondence and the projected image is converted into electrical signals. A TFT drive image sensor belonging to the category of this type of the image sensor has been used. In the TFT drive image sensor, the projected image is divided into a great number of picture elements (photodetecting elements), electric charges generated in the photodetecting elements are temporarily stored every specific block of photodetecting elements in the capacitors existing between the wires by using thin film transistor switching elements, and the charges are time-sequentially read out of the capacitors in the form of electrical signals at a speed of several hundreds kHz to several MHz. In the TFT drive image sensor, the image can be read using a single drive IC, through the action of the TFTs. Therefore, the number of drive ICs for driving the image sensor can be reduced.
The TFT drive image sensor, as shown in FIG. 1 showing an equivalent circuit of the image sensor, is made up of a linear photodetecting element array 51 having a length substantially equal to the width of a document, a charge transfer section 52 consisting of a plurality of thin film transistors Ti,j (i=1 to N, j=1 to n) provided in association with photodetecting elements 51" in a one-to-one correspondence, and a matrix-arrayed multilayered wiring structure 53.
The photodetecting element array 51 is divided into photodetecting element groups of N number of blocks. The "n" number of photodetecting elements 51" forming one group may be equivalently expressed by photo diodes Pi,j (i=1 to N, j=1 to n). The photodetecting elements 51" are respectively connected to the drain electrodes of the thin film transistors Ti,j. The source electrodes of the thin film transistors Ti,j are respectively connected, every group of photodetecting elements in the block, to the "n" number of common signal lines 54 through the multilayered wiring structure 53. The signal lines 54 are connected to a drive IC 55.
The gate electrodes of the thin film transistors Ti,j are connected to a gate pulse generator 56 so that, the transistors are turned on every block. The electric charges generated in the photodetecting elements 51" are stored, for a predetermined period of time, in the parasitic capacitors of the photodetecting elements and the overlap capacitors between the drain and gate electrodes of thin film transistors. Then, the charges are sequentially transferred every block to the wiring capacitors Ci (i=1 to n) of the multilayered wiring structure 53. During the charge transfer, the thin film transistors Ti,j serve as charge transfer switches.
A gate pulse .PHI.G1 is transferred from the gate pulse generator 56 through the gate signal lines Gi (i=1 to n) to the thin film transistors T1,1 to T1,n of the first block, to turn them on. The charges generated in the photodetecting elements 51" of the first block are transferred to and stored in the wiring capacitors Ci. By the charges stored in the wiring capacitors Ci, the potentials in the common signal lines 54 vary. The varied voltages are time-sequentially introduced onto an output line 57 by successively turning on analog switches SWi (i=1 to n) in the drive IC 55.
In response to the gate pulses .PHI.G2 to .PHI.Gn, the transistors T2,1 to T2,n to TN,1 to TN,n in the second to N-th blocks are turned on, so that the charges of the photodetecting elements are transferred every block, thereby obtaining an image signal for one line in the main scan direction on an original document. The original is moved by means of a document feed means (not shown), such as rollers, and the sequence of the operations as stated above is repeated. Finally, image signals of the whole document are obtained (Japanese Patent Unexamined Publication No. Sho. 63-9358).
The construction of the matrix-arrayed multilayered wiring structure 53 is shown in FIGS. 2 and 3. FIG. 2 is a plan view showing the construction of the wiring structure, and FIG. 3 is a cross sectional view showing the same. As shown, the multilayered wiring structure 53 is constructed with a substrate 21, a lower layer signal line 31, an insulating layer 33, and an upper layer signal line 32. In the structure, the signal line 31, the insulating layer 33 and the signal line 32 are multilayered on the substrate 21. The signal lines 31 and 32 are arranged crossing each other. Contact holes 34 are provided for interconnection of the upper and lower signal lines.
As described, in the construction of the conventional image sensor, the multilayered wiring structure has the matrix construction such that the upper and lower signal lines cross each other with the insulating layer 33 being interlayered therebetween, as shown in FIG. 3. Accordingly, a coupling capacitor exits at each cross portion of the lower and upper layer signal lines 31 and 32. The coupling capacitor causes a potential difference between the upper and the lower lines at each cross portion. The output signal of one of the signal lines is influenced by the output signal of the other. Cross talk occurs thereat, the charges are mistakenly read, and hence the reproduction for gradation in the image sensor is deteriorated.
The image sensor as described above is a monochromatic image sensor. A TFT drive color image sensor having the matrix-arrayed multilayered wiring structure also has similar problems.
The construction and operation of the TFT drive color image sensor having the matrix-arrayed multilayered wiring structure will be described with reference to FIG. 4 showing an equivalent circuit of the color image sensor. In FIG. 4, like or equivalent portions are designated by the same reference symbols as in FIG. 1.
As shown in FIG. 4, in the color image sensor, "n" number of sandwich type photodetecting elements (photo diodes P) arrayed on an insulating substrate made of glass, for example, are grouped into a block. N number of blocks make up a photodetecting element array 51 (consisting of photo diodes P1,1 to PN,n). Three lines of photodetecting element arrays 51a, 51b and 51c are arranged in the subsidiary scan direction. The array 51a is coupled with a red (R) passing filter. The array 51b is coupled with a green (G) passing filter. The array 51c is coupled with a blue (B) passing filter. The color image sensor further includes charge transfer section 52 consisting of thin film transistors T1,1 to TN,n connected to the photodetecting elements 51", a matrix-layered multilayered wiring structure 53, "n" number of common signal lines 54, which are led from the charge transfer section 52 through the multilayered wiring structure 53, corresponding to each group of the photodetecting elements of each block, and analog switches SW1 to SWn provided in a drive IC 55 and connected to the common signal lines 54.
The first ends of the photodetecting elements 51" receive voltages VB1, VB2, and VB3 through common electrodes provided for the photodetecting element arrays. The wires, which are led from the source electrodes of the thin film transistors (TFTs) of the charge transfer section 52, which are connected to the photodetecting elements 51" in the array 51a of the first line, are connected to the source electrodes of the thin film transistors connecting to the photodetecting elements 51" in the arrays 51b and 51c of the second and third lines. The wires are connected as common wires to the matrix-layered multilayered wire structure 53, that is, to the common signal lines 54 whose number is equal to that of the photodetecting elements 51" in the block of the photodetecting element array 51. The gate electrodes of the thin film transistors of the photodetecting element arrays 51 are connected every block, and accordingly gate terminals GR1 to GRN, GG1 to GGN, and GB1 to GBN are provided corresponding to the three lines of the photodetecting element arrays 51a, 51b and 51c.
A method of driving the color image sensor will be described. When an original document (not shown) put on the photodetecting element arrays 51 is illuminated with light from a light source (not shown), the reflected light illuminates the photodetecting elements 51" (photodiode P) of the photodetecting element arrays of the respective lines, to cause the photodetecting elements to generate charges according to the light and shade, and color on the original. The generated charges are stored in the parasitic capacitors of the photodetecting elements 51" and the overlap capacitors between the drain and gate electrodes of TFTs. The photodetecting element arrays 51a, 51b, and 51c are coupled with the color filters allowing the specific colors (red, green, and blue) to pass therethrough. Accordingly, the first photodetecting element array 51a responds to red to generate charges; the second photodetecting element array 51b responds to green to generate charges; the third photodetecting element array 51c responds to blue to generate charges.
When the TFT is turned on in response to a gate pulse .PHI.G from a gate pulse generator (not shown), the photodiode P is connected to the common signal line 54, so that the charges stored in the parasitic capacitor and the like are transferred to and stored in wiring capacitors C1 to Cn of the multilayered wiring 53. More specifically, when charges are generated in the photo diodes P1,1 to P1,n in the first block of the first photodetecting element array (responsive to red) 51a, the transistors T1,1 to T1,n are turned on in response to a gate pulse .PHI.G1 from the gate pulse generator. Charges generated in the photo diodes P1,1 to P1,n are transferred to and stored in the wiring capacitors C1 to Cn in the matrix-arrayed multilayered wiring structure 53. Afterwards, the transistors T1,1 to T1,n are turned off. In the above case, since the wiring capacitor (Ci) is 100 times as large as the parasitic capacitor of the photodiode P in capacitance, it is not necessary to reset the photodiode P.
A timing generator (not shown) successively applies read switching signals .PHI.s1 to .PHI.sn to the switches SW1 to SWn in the drive IC 55. After one timing, the timing generator likewise applies reset switching signals .PHI.R1 to .PHI.Rn to reset switching elements RS1 to RSn of the drive IC. Consequently, the charges stored in the wiring capacitors C1 to Cn are outputted (COM) in the form of image signals. Subsequently, charges generated in the photo diodes in the next block are transferred. Following the read operation the first photodetecting element array (responsive to red) 51a, the read operation is performed for the second photodetecting element array (responsive to green) 51b, and finally the read operation is performed for the third photodetecting element array (responsive to blue) 51c.
The image signals (image data) read from the first to third photodetecting elements 51a to 51c are stored into a memory outside the image sensor. The pitches of the photodetecting elements of the photodetecting element arrays are calculated and the image data are composed on the calculation result.
As described above, also in the TFT drive color image sensor with a matrix-arrayed multilayered wiring structure, cross talk occurs thereat, the charges are mistakenly read, and hence the reproduction for gradation in the image sensor is deteriorated.